Recently, a variable valve timing control apparatus that allows opening and closing timing of an intake valve and an exhaust valve to be changed in accordance with an operation condition of an internal combustion engine (which will be hereinafter referred to also as an engine) is practically in use. The above-mentioned variable valve timing control apparatus includes a mechanism, for example, for changing a relative rotational phase of a driven side rotating member relative to a rotation of a driving side rotating member driven to rotate by an operation of the engine, and thereby changing the opening and closing timing of the intake/exhaust valve opened and closed by a rotation of the driven-side rotational member.
Generally, an optimal opening and closing timing of the intake/exhaust valve varies depending on the operation condition of the engine including, for example, a time when the engine starts up and a time when a vehicle runs. Thus, when the engine starts up, the opening and closing timing of the intake/exhaust valve which is optimal for the engine to start up is realized by restraining the relative rotational phase of the driven side rotating member relative to the rotation of the driving side rotating member at a predetermined phase. In addition, a tapping noise, which is generated when a partition portion of a fluid pressure chamber formed by the driving side rotating member and the driven side rotating member moves within the fluid pressure chamber in a circumferential direction of the rotating members, is restricted from occurring. Consequently, before the engine is stopped, the relative rotational phase needs to be restrained at the predetermined phase.
Among lock mechanisms for restraining the relative rotational phase at the predetermined phase, a lock mechanism is known, which includes, for example, a locking member and a coil spring providing a biasing force to the locking member which are accommodated in one of the driving side rotational member and the driven side rotational member, and includes a locking hole provided at the other one of the driving side rotational member and the driven side rotational member. According to this lock mechanism, the locking member is biased by the biasing force to protrude so as to engage with the locking hole so that a locked state is established. The locking member is pushed to retract from the locking hole by a pressure, which is higher than the biasing force, of a working fluid (which will be hereinafter referred to also as a working oil) so that an unlocked state is established.
A known control apparatus of the internal combustion engine, which includes the above-described lock mechanism is disclosed in JP2011-1888A (which will be hereinafter referred to as Patent reference 1). The known control apparatus of the internal combustion engine disclosed in Patent reference 1 is provided with a lock mechanism for locking a relative rotational phase of an inner rotor relative to an outer rotor at a predetermined intermediate phase that is positioned between a most retarded angle phase and a most advanced angle phase. During idling, the relative rotational phase is in the most regarded angle phase. In this state, in a case where an ignition switch is turned off, electricity is supplied to an oil switching valve and thus working oil is not supplied to a hydraulic chamber-for-unlocking. After this, an oil control valve is switched so that the relative rotational phase changes in an advanced angle direction. In a case where the relative rotational phase is in the predetermined intermediate phase, a locking key is biased by a biasing force of a spring to engage with the hydraulic chamber-for-unlocking and locked thereat, so that the relative rotational phase of the outer rotor and the inner rotor relative to each other is locked.
It is judged by an intermediate position fixing determination means whether or not the relative rotational phase of the outer rotor and the inner rotor relative to each other is locked. In a case where it is judged that the relative rotational phase is locked, ignition is turned off and the engine stops. At the same time as the ignition is turned off, the supply of electricity to the oil switching valve is stopped.
According to Patent reference 1, an oil pump is operated by rotation of the engine so that the working oil is supplied. Thus, the oil pump supplies the working oil during a period from a time at which a rotation speed of the engine starts to decrease due to the turning off of the ignition until a time at which the engine stops, and therefore hydraulic pressure is generated. The supply of electricity to the oil switching valve has already been stopped at this time, and thus the working oil is supplied from the oil pump to the oil pressure chamber-for-unlocking, so that the hydraulic pressure works on a pressure-receiving surface of the locking key. In a case where the hydraulic pressure working on the pressure-receiving surface exceeds a hydraulic pressure for unlocking, the lock is released. Once the lock is released, the relative rotational phase changes in a direction which allows the working oil to be supplied from the oil control valve or in a direction which generates an average cam torque (generally, a retarded angle direction), and thus the engine may not be started at an optimal opening and closing timing of the intake/exhaust valve.
A need thus exists for a variable valve timing control apparatus which is not susceptible to the drawback mentioned above.